No one checks the return value of debugfs_create_atomic_t(), as it's not
needed, so make the return value void, so that no one tries to do so in
the future.

Link: https://lore.kernel.org/r/20191016130332.GA28240@kroah.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>